A known method of measuring the time tp taken by a signal, e.g. An ultrasound signal, to propagate between two transducers consists in exciting the emitter transducer with an excitation pulse ie1. Such an excitation pulse is substantially in the form of a squarewave and the frequency spectrum includes the excitation frequency of the transducer. On being emitted by the emitter transducer, this pulse gives rise to an ultrasound wave in the medium between the two transducers. This wave will propagate towards the receiver transducer. FIG. 1 shows the excitation signal ie1 of the emitter transducer and the signal sr1 as output by the receiver transducer. The method consists in detecting the first oscillation of said wave on arrival at the receiver transducer. The propagation time tp is then the time between the instant at which the emitter transducer is subjected to the excitation pulse and the instant at which the first oscillation of the ultrasound wave is detected as arriving at the receiver transducer. That method is particularly difficult to implement and suffers from inaccuracy that gives rise to an erroneous measurement of propagation time. At the receiver transducer, the ultrasound wave gives rise to a response signal of very low amplitude. By way of example, in the context of an ultrasound flow meter used in heating networks, for a transducer having a resonant frequency close to 10 megahertz (mhz), the amplitude response of a received signal corresponds to a value lying in the range about 3 millivolts (mv) to 10 mv. FIG. 2 shows the appearance of the response signal from the receiver transducer sr1 when the emitter transducer is excited by a single pulse. The method consists in detecting the first oscillation of the ultrasound wave pf1 by detecting when a voltage threshold is crossed. That method requires very low voltage levels to be detected and very accurate control over the trigger threshold of the device for detecting the arrival of an oscillation in order to avoid introducing any delay in the propagation time measurement. That method can be made to be accurate by using an electronic threshold trigger component that is of high performance, but expensive. However, it becomes inaccurate when using an electronic threshold trigger component of ordinary type.
U.S. Pat. No. 5,123,286 discloses a method of determining the propagation time of an ultrasound wave between two transducers. The emitter transducer is excited by a squarewave pulse which gives rise to the appearance of a response signal that is typical for a damped oscillator whose peak amplitude increases over a certain number of periods before decreasing. That method proposes determining the propagation time between the instant at which the emitter transducer is excited and the instant at which the ultrasound signal is received by the receiver transducer. It consists in calculating an envelope for the response signal by determining firstly the amplitude of a group of periods and secondly the instants of the zero crossings of said periods. The point where said envelope intersects the baseline of the response signal is then calculated in order to determine the instant at which the response signal appears at the transducer. Finally, the propagation time is determined by calculating the difference between the excitation instant and said instant at which the signal appears.
Document DE 4 017 022 discloses electronic apparatus for improving the accuracy with which propagation time of an ultrasound signal between two transducers is measured. That apparatus proposes determining the instant corresponding to reception of the ultrasound signal in precise manner. The receive signal is applied to two comparators whose threshold voltages are different. A “cycle” signal and a “period” signal are generated. These signals trigger a cycle length counter and a period length counter. The output from the cycle length counter is connected to a memory for storing a binary signal corresponding to the receive signal. At the end of measuring cycle length, the content of the memory is analyzed while taking the period of the signal into consideration. The circuit takes account of the stored value to correct the length of the cycle and to determine propagation time.
An ultrasound flow meter described in document U.S. Pat. No. 5,777,238 measures the propagation time of the ultrasound signal by using at least one, adaptive or dynamic, reference windowing signal (which signal comprises a fixed portion and a variable portion) and a zero crossing detector or circuit. A zero crossing is determined for each period making up the receive signal and the propagation time is determined on the basis of a mean calculated from the times corresponding to said zero crossings.
Those methods are complex to implement, and require various measurements to be made and stored, and they also require numerous calculations to be performed.